Protein-rich foods or drinks, which have been taken by athletes for protein supplementation, are increasingly demanded in recent years because now they are also often taken by elderly people for nutritional supplement and by ordinary consumers as a substitute for a diet. Most of these protein foods or drinks are concentrated in a neutral pH range, thus the flavor thereof tends to be monotonous. Those having an acidic flavor can broaden the taste variety, but there are not so many of them because many of protein are slightly soluble under acidic conditions.
Examples of known protein-containing acidic foods or drinks include fermented milk beverages and other sour milk beverages. However, these protein-containing acidic foods or drinks have “astringency” that is a peculiar smell or an unpleasant taste, thereby giving an unpleasant feeling that is often expressed by such terms as an astringent taste, a filmy mouthfeel and graininess. In these protein-containing acidic foods or drinks, protein that has been insolubilized and flocculated (or aggregated) by an acid, heating, fermentation or the like is in a dispersed state, and the flocculate (or aggregate, hereinafter simply referred to as flocculate) is recognized as a cause of astringency. Therefore, it has been attempted to relieve the astringency by reducing the particle size of the flocculate (Patent Document 1), or by removing the flocculate (Patent Document 2). Further, Patent Document 3 discloses a process for preparing a beverage with relieved astringency. said beverage comprising fermented whey, soybean milk, and pectin or xanthan gum as a stabilizer, and Patent Document 4 discloses a method, wherein propylene glycol alginate and a metallic salt is added as modifiers having an astringency relieving effect after homogenizing fermented soybean milk. However, either of these methods is not directed to the reduction of astringency developed during eating or drinking dissolved protein.
There are acidic foods or drinks containing dissolved protein at a high concentration, while their numbers are very limited. They are beverages and jelly beverages containing a whey protein isolate (WPI) in an amount of 3 to 9%, and are characterized by their transparency. The present inventors have previously found a process for producing a soybean protein that shows excellent solubility under acidic conditions (Patent Document 5), and have succeeded in preparing protein-rich acidic foods or drinks using the protein. However these acidic protein foods or drinks were unexpectedly more astringent rather than aforementioned acid milk beverages. This phenomenon contradicts the conventional recognition that astringency is derived from the flocculate of protein. Although such astringency of acidic foods or drinks containing dissolved protein can be relieved depending upon a protein concentration or a processing method, no decisive improvement measure has yet found.
Patent Document 1: JP 53-104764 A
Patent Document 2: JP 10-295270 A
Patent Document 3: JP 62-111632 A
Patent Document 4: JP 54-84068 A
Patent Document 5: WO 02/067690 A1